1. Field of the Invention
The present invention relates to a resolver which detects an angular position of an object. The present invention also relates to a motor including such a resolver.
2. Description of the Related Art
Hydraulic power steering is a known system which assists the driver of a vehicle. This system assists the driver with a hydraulic pressure generated by a pump driven by the vehicle's engine. Another system, electric power steering (hereinafter, simply referred to as “EPS”), which provides assistance by driving a motor by the vehicle's battery, has been increasingly used in recent years. The EPS can reduce loss of engine power and is more efficient as compared with hydraulic power steering, and therefore has attracted attention.
The EPS requires an accurate angle detection device which can measure a rotational angle of a motor with high accuracy in order to provide highly accurate control. Moreover, since vehicles are required to precisely work for long periods in various environments, the angle detection device for EPS as one component of the vehicle is required to be highly reliable.
For this reason, a variable reluctance resolver has been used as the angle detection device for EPS because it has high detection accuracy and has higher environmental stability than optical encoders and magnetic encoders. The variable reluctance resolver is arranged so that permeance of a gap formed between a resolver rotor and a resolver stator is sinusoidally changed with rotation of the resolver rotor, and, based on this change, detects the angular position of the resolver rotor or measures the rotation angle thereof. For example, the resolver rotor is arranged in such a shape that the gap permeance is sinusoidally changed with rotation of the resolver rotor. Thus, the shape of the resolver rotor largely affects the detection accuracy of the resolver.
The resolver rotor is usually defined by a plurality of rotor pieces stacked on one another, each of which is formed by punching out an electromagnetic steel plate. The rotor pieces are joined to each other by bonding, deformation, or molding, for example.
In a case where the rotor pieces are joined to each other by being covered by a protection cover formed by resin molding, however, the protection cover increases the manufacturing cost.
In a case where the rotor pieces are joined to each other by adhesive or the like, the adhering force is lowered at some temperatures, causing the rotor pieces to peel off. When the rotor pieces peel off, each rotor piece is displaced and therefore the rotation angle cannot be measured accurately. In order to prevent this, it is necessary to sufficiently consider the endurance of the adhesive. However, this consideration narrows the range of choices for the adhesive material, increasing the manufacturing cost.
In a case of joining the rotor pieces to each other by deformation, a sufficient level of joining strength can be obtained and high reliability can be maintained in various environments. However, deformation may adversely affect the outer shape of the resolver rotor, thus lowering the detection accuracy.